It has been known to form poly(arylene sulfides) represented by poly(p-phenylene sulfide) into biaxially-stretched films. These poly(arylene sulfides) and poly(p-phenylene sulfide) may hereinafter be called "PAS" and "PPS" respectively. Incidentally, in order to make a film useful as an industrial film, it is necessary to reduce the coefficient of dynamic friction while controlling the surface roughness within a predetermined range and hence retaining planarity, in other words, to impart high lubricity. The friction coefficient, namely, slip properties between plastic films themselves or between a plastic film and another material generally give significant influence to the efficiency of work when the plastic films or film is processed. When coating, printing or wrapping is performed using a film having poor slip properties, a large tensile force is applied to the film so that difficulties are encountered in feeding the film or taking it up into a roll. When the film is employed as a capacitor film in particular, the film is processed through many take-up and rewinding steps in the course of its assembly into a capacitor element. If the slip properties of the film are poor, the film may be torn, stretched and whitened, and/or caused to develop wrinkles. Due to these problems, the film cannot be assembled successfully into a capacitor element or even if it looks as if assembled into a capacitor element, the capacitor element involves one or more fatal defects and cannot be used practically as a capacitor element. On the other hand, unduly good slip properties may also deteriorate the efficiency of work in some instances, including the occurrence of irregular winding by way of example. It is therefore necessary to improve slip properties of a film, in other words, to render the film slidable easily to a certain extent. These easily slidable properties will hereinafter be called "high lubricity" for the sake of brevity. For PPS films, it is also essential to impart high lubricity from the practical viewpoint. It has heretofore been known to control the surface roughness of a film by adding, as a filler, one or more inert inorganic materials in the form of fine powder, for example, calcium carbonate, silica, alumina, carbon black, glass, calcium phosphate and/or the like (Japanese Patent Publication No. 5101/1984). The addition of such an inorganic filler however results in a reduction in the dielectric strength of a PPS film, thereby making the film somewhat insufficient for use as a capacitor base material or electrically-insulating base material in view of the standard of properties required presently for such an application.
The present inventors have already proposed to add poly-4-methylpentene-1 in place of a conventional inorganic filler with a view toward imparting high lublicity to PAS (Japanese Patent Application No. 75827/1986 ; Japanese Laid-Open Publication No. 232442/1987 ; U.S. Pat. No. 4731390 ; Canadian Patent Application No. 533626 ; European Patent Application No. 87302846 8). The resultant composition has however been found not to be fully satisfactory in process-ability such as extrusion stability.
On the other hand, polystyrene films have excellent electrical properties and are used as insulating films, especially, as capacitor films. They however have a drawback that their heat resistance is inferior.
It has also been known to blend a polystyrene-based resin with PAS in order to modify the latter resin, as disclosed in Japanese Patent Publication No. 13469/1978 by way of example. However, the PAS employed in the above patent publication is such that could be obtained in accordance with a process disclosed, for example, in Japanese Patent Publication No. 3368/1970, namely, by reacting a polyhalogenated aromatic compound and an alkali metal sulfide at an elevated temperature in a polar organic medium. PAS obtained by such a process has an extremely low polymerization degree and a low melt viscosity, so that it cannot be formed into biaxially-stretched films, to say nothing of its extrusion into films. In addition, the PPS of the low polymerization degree still cannot provide useful films even when it is crosslinked or cured in order to permit its injection. The resin composition described in Japanese Patent Publication No 13469/1978 has been provided with a view toward improving the moldability and impact resistance of PAS without lowering its tensile property The resin composition is said to be moldable into articles for various application fields. Its formation into films is however not disclosed there.
It has also been known to blend one or more of various synthetic resins with PAS having a high molecular weight in general. It has however been unknown to form a resin composition, which is composed of PAS and a polystyrene-based resin, into a biaxially-stretched film.
As has been described above, it has not been possible to avoid the reduction of dielectric strength characteristics so long as high lubricity is imparted to a PAS film by the addition of an inorganic filler. Furthermore, it has not been known to form a resin composition, which is composed of PAS and a polystyrene-based resin, into a biaxially-stretched film.
In the meantime, the present inventors found that a resin composition formed by blending a polystyrene-based resin with a substantially linear, high molecular weight PAS surprisingly has good film-forming properties, permits biaxial stretching, has high lubricity without need for addition of an inorganic filler and can provide a biaxially-stretched film having superb dielectric strength characteristics. An application has already been filed on that subject matter, to which Japanese Patent Application No. 116777/1987 (U.S. Patent Application Ser. No. 191131 filed May 6, 1988 ; European Patent Application No. 88304309.3) has been allotted. Although the biaxially-stretched film has high lubricity and is excellent in dielectric strength characteristics, its surfaces tend to become somewhat too rough. In addition, PAS films generally contain many so-called fish eyes. This tendency cannot however be improved to any significant extent even by the blending of the styrene-based resin.